The human immunodeficiency disease X-linked a gamma globulinemia (XLA) is caused by mutations in Bruton's tyrosine kinase (Btk) but it is not clear how defective expression of this kinase leads to low serum immunoglobulin levels and blocks in B lymphocyte development. Mice with a point mutation in Btk have produce a murine model for studying x- linked immunodeficiency, but the phenotype of the disease in mice is less severe than in man. Xid mice produce circulating B lymphocytes, albeit at lower levels than in normal mice, and the block in B cell development is at the immature B cell stage instead of the pro-B to pre-B cell stage, as occurs in man. The reasons for the differences in presentation of the disease in mouse and man are unknown, but could be due to intrinsic differences in B cell development or to differential interactions of Btk with additional proteins. Our studies in mice indicate that the transcription factor Bright (B cell regulator of immunoglobulin heavy chain transcription) interacts with Btk. Bright is a 70 kDa protein that binds to promoter and enhancer sequences of the immunoglobulin heavy chain locus where it has been associate with increases in transcription. Although sequences of the immunoglobulin heavy chain locus where it has been associated with increases in transcription. Although xid cells can express Bright protein, they are deficient in Bright DNA-binding activity. Furthermore, while Bright and Btk co-precipitate from control B lymphocytes, they do not appear to interact in xid cells. Little is known about human Bright, but our preliminary data suggest that it also interacts with Btk and that it can be induced in normal peripheral blood lymphocytes. However, we have observed a number of differences between mouse and human Bright-like proteins. To learn more about human Bright and its potential role in XLA, it will be necessary to evaluate Bright expression and interactions with Btk. We propose to utilize the information produced from the other three projects to assess Bright's role in human lymphopoiesis. The specific aims proposed are: (1) to define where Bright is expressed during human lymphocyte development by RT-PCR, Western blotting and mobility shift assay of sorted subpopulations, (2) to define the molecular interactions that occur between human Bright and Btk using co-precipitation, plasmon resonance, co-transfection and directed yeast two-hybrid assays, (3) to determine how XLA mutations affect these interactions using the same techniques, and (4) to identify human gene targets for Bright using classical methods and antibody facilitated cloning. These studies will yield important new information about human Bright and its interactions with Btk, and may also lead to insights into B cell development and XLA.